1. Field of the Invention
The present invention relates to a process controller having a combination of feedback and feedforward control systems and, more particularly, to a process controller having an improved auto-tuning means for feedforward control due to a variation in load.
2. Description of the Prior Art
The present invention is an improvement of U.S. Pat. No. 4,714,988 (Hiroi et al.) filed by the present applicant on Dec. 22, 1987, and incorporates all the disclosure contents of the above Patent.
In a variety of manufacturing plants, it is most important to achieve a highly-flexible productivity, energy saving, and high-quality products. For this purpose, the plants must be controlled by optimal control parameters. However, in a control system being subjected to a large change in the influence of a disturbance or the like, it is very difficult to select optimal control parameters. Thus, demand has arisen for an auto-tuning means which can relatively easily and automatically tune optimal control parameters in a control system whose control parameters are difficult to select.
With the aim of satisfying this demand, a process controller having an auto-tuning function of a feed-forward model has been developed, and is shown in FIG. 5 of U.S. Pat. No. 4,714,988. This apparatus will now be described in conjunction with the reference numerals used in the above Patent.
This process controller comprises feedback and feedforward control systems. In the feedback control system, comparator 31 calculates deviation en in accordance with target variable SV and process variable X obtained by measuring controlled variable X. Deviation en is supplied to adjusting unit 32, which performs a speed PID operation. The PID output from adjusting unit 32 is then converted to position signal A by speed-position signal converter 34 through adder 33. Position signal A is supplied to adder 35, in order to obtain manipulation signal M. Manipulation signal M, once obtained, is supplied to object 36 to be controlled, thus adjusting controlled variable X.
In the feedforward control system, multiplier 40 derives feedforward controlled variable B from disturbance signal D. Thereafter, a variation in the control system, caused by disturbance signal D, is compensated for by static and dynamic characteristic compensation means.
The static characteristic compensation means comprises difference arithmetic unit 51, switch 62B, and the like. Feedforward controlled variable B, obtained by multiplier 40, is converted to speed signal C by difference arithmetic unit 51. Speed signal C is then supplied to adder 33, through switch 62B1. The dynamic characteristic compensation means comprises incomplete differentiator 42, switch 62B, and the like. Differentiator 42 derives change E in dynamic characteristics from feedforward controlled variable B, and supplies it, through switch 62B2, to adder 35.
In the process controller described above, a parameter (coefficient) for the feedforward control system is determined as follows:
The auto-tuning control means comprises subtractor 43, which derives a deviation signal, between feed-forward controlled variable B and output signal A, from speed-position signal converter 34. In addition to the deviation signal, the auto-tuning control means derives control deviation signal en, static characteristic compensation signal C, and dynamic characteristic compensation signal E, and supplies these signals to corresponding signal level detectors 55 to 59. Signal level detector 55 outputs an ON signal (H) when the deviation signal exceeds a predetermined range, while, in contrast, the other signal level detectors 57 to 59 output ON signals (H) when their input signals (C, E, en) fall within the predetermined range. When the ON signals are output from all signal level detectors 55 to 59, a start control signal is output from an AND circuit (60, 61), thereby starting a timer circuit (62-64). After a predetermined period of time has passed from the starting of the timer circuit, the timer circuit outputs a tuning timing signal to turn on switch 62A. When switch 62A is turned on, the deviation signal is input, through switch 62A, to feedforward parameter correction arithmetic unit 44 in order to determine a parameter for feedforward control. Thus, tuning control is performed on the basis of this parameter. More specifically, feedforward parameter correction arithmetic unit 44 corrects a feedforward reference parameter using an integral value of the deviation signals, and a predetermined coefficient, and outputs the obtained parameter to multiplier 40. The feedforward control parameter is tuned (automatically tuned) so that the deviation signal from subtractor 43 becomes zero. During parameter tuning, switches 62B1 and 62B2 are kept off, for the purpose of suppressing a variation in manipulated variable, caused during tuning of the feedforward parameter.
However, the auto-tuning control means described above fetches a number of operating state signals (C, E, en, etc.) from respective sections of the apparatus, and automatically tunes the parameter under the stable conditions, and, in some plants, it is difficult to obtain an auto-tuning timing. If the stable conditions are moderated, tuning accuracy of the parameter is largely degraded.
A large number of functions are added to obtain an auto-tuning timing, resulting in a very complicated arrangement. In addition, many loop controllers adopt distributed type controllers. In this case, in order to realize a compact apparatus and function dispersing, a memory capacity is limited, and hence, the apparatus cannot have so many functions. If the apparatus is designed to have all the functions of that shown FIG. 5 in U.S. Pat. No. 4,714,988, a large volume of calculation processing, and processing data therefor must be stored, thus requiring a considerably large memory capacity. Nevertheless, it is still difficult to design the apparatus to have all these functions.